This invention relates to permanent magnetic materials of RExe2x80x94TMxe2x80x94B alloys, and, more particularly, to methods of producing alloyed Nd/Prxe2x80x94Fexe2x80x94B alloyed powders.
This invention relates to magnetic Materials comprising one or more transition metals, one or more rare earth elements and iron. A variety of such materials are known. See, for example, U.S. Pat. Nos. 4,374,665, 4,402,770, 4,409,043 and 4,533,408 of Koon which disclose magnetic alloys comprising iron, lanthanum and a lanthanide, and boron; U.S. Pat. Nos. 4,802,931 and 4,851,058 to Croat which disclose single phase TMxe2x80x94RExe2x80x94B alloys, U.S. Pat. No. 4,935,074 of DeMooij et al. which discloses a material including iron, a relatively high percentage of boron, and Nd and/or Pr; U.S. Pat. No. 5,071,493 to Mizogochi et al., and U.S. Pat. No. 4,770,723 to Sgawa et al. Methods of preparing such magnetic materials are taught in U.S. Pat. Nos. 4,867,785 and 5,116,434 to Keem et al., and U.S. Pat. Nos. 4,715,891 and 4,753,675 to Ovshinsky et al.
In general, the magnetic material is prepared in two-steps. The first step is to prepare a substantially homogeneous master alloy of the desired composition of the various components, and then to break the alloy into relatively large, e.g., 1 cm pieces, that can be easily remelted for later processing. The second step is to remelt the alloy in a crucible, and then to quench and rapidly solidify (e.g., 25-30 micron and not over 50 microns thick) homogeneous ribbon. Typically the quenching and rapid solidification is accomplished by passing the melted material through a small (e.g., 1 mm in diameter) orifice onto a moving chill surface, e.g., a rotating chill surface such as that shown in aforementioned U.S. Pat. No. 4,867,785.
Typically, the ribbon is then crushed into fine powder, the powder is then heat treated and magnetically separated (e.g., as disclosed in U.S. Pat. No. 5,116,434) to separate powder having high magnetic parameters from that having lower magnetic parameters, and the high magnetic parameter powder is made into permanent magnets.
The above described procedure, and particularly the quenching and rapid solidification, is expensive and subject to a number of potential problems and difficulties. For example, unsatisfactory product may result from variations in the flow from the crucible caused by alloy/crucible reaction products, slag or products of reaction between the atmosphere and the molten alloy at the crucible orificd that can clog or impede flow through the small crucible orifice, or for relatively small variations in the chill surface temperature or speed that affect the quench rate and/or ribbon thickness.
There has been, and remains, a need for a process that is less expensive and has fewer difficult-to-control parameters.
According to the present invention, a class of alloyed magnetic materials, i.e., materials having a relatively high boron content, are prepared in a single step procedure. A molten mixture of the desired composition is cooled at a rate slower that about 105 degrees Celsius per second, and preferably more than about 104 degrees Celsius per second. Without the need for conventional rapid quenching, a powder of the slow-cooled material may then be heat treated, and, optionally, magnetically separated. Surprisingly, such slow cooled material has been found to have magnetic properties superior to those of material of identical composition made in the conventional two-step, rapid quenching procedure.
In some preferred practices of the invention, the molten mixture is solidified by depositing it on a heat conducting surface such that it forms a layer between about 120 and about 300, and preferably between about 120 and about 150, microns thick, and the relatively thick layer of slow-cooled material is then crushed into powder before heat treating. In these and other preferred practices of the invention, the molten mixture contains more than about 10 (and most preferably not less than about 20) atomic percent boron and not more than about 10 atomic percent of a rare earth. Preferably at least 90 percent by weight of the rare earth content is Nd, Pr or a mixture thereof and any other rare earth constitute less than 10 percent of the total rare earth content. In most preferred practices, the molten mixture includes at least 60 atomic percent, and preferably at least 70 atomic percent, Fe.